LSI (Large Scale Integration) circuits are semiconductor integrated circuits which have over 1000 elements, and ULSI or the like are those which have over one million elements stored on a single chip, and in recent years, 100 million or more elements have been formed on a single chip.
This type of ULSI cannot be accommodated simply by miniaturizing elements on a plane, so multiple layer wiring construction with a plurality of overlapping layers of wiring has become essential. A multiple layer wiring construction increases the capacitance between layers and causes the elements to have longer signal delay times, so in order to reduce the capacitance between layers, a low dielectric constant interlayer insulating film (hereinafter referred to as Low-K film) is used as an insulating film between layers.
Incidentally, the ashing process that removes the unneeded resist mask after patterning conventionally uses oxygen (O2) plasma. However, the dielectric constant of Low-K films is dramatically increased by ashing with oxygen plasma. In particular, interlayer insulating films which use porous materials with a low relative dielectric constant have a plurality of fine air voids which are exposed (large relative surface area), so the resistance to highly reactive oxygen plasma is extremely low, the film properties are easily degraded, and the degradation due to oxygen plasma is very significant.
Therefore, technology has been developed to prevent degradation of a Low-K film by performing ashing using plasma produced from a gas blend containing nitrogen and hydrogen but no oxygen (Refer to Patent Reference 1), or to perform ashing using an oxygen-free plasma from a gas containing hydrogen and a gas containing fluorine (Refer to Patent Reference 2).
Patent Reference 1: Japanese Laid-open Patent Application 2002-261092
Patent Reference 2: Japanese Laid-open Patent Application 2001-110775
With the wafer ashing process when the aforementioned Low-K film is used as an interlayer insulation film, removing the resist by conventional ashing using oxygen (O2) plasma is not suitable, so ashing gas conditions that are completely different from conventional are required, and plasma processing has been performed using hydrogen (H2), nitrogen (N2), or ammonia (NH3) and the like. However, there are problems with the processing capability of the plasma processing equipment when these ashing gas conditions are used.
In other words, with a gas blend of nitrogen and hydrogen disclosed in Patent Reference 1, as shown in FIG. 4 for example, the ashing rate is high, but as shown in FIG. 3, there is a large increase in the dielectric constant. Furthermore, with a gas blend of gases containing hydrogen and gases containing fluorine according to Patent Reference 2, there is a dramatic increase in the dielectric constant and the film properties may be degraded.